Increasing the density of iron powder by alternate rolling and grinding



Sept. 1, 1959 s. R. cRooKs ETAL 2,902,357 INCREASING THE DENSITY OF IRONPOWDER BY ALTERNATE ROLLING AND GRINDING Filed June 11, 1954 ifDUCE 44pea/v COMPOUND 70 Mfr/14L /0 mm GEM/D 70 POM 55? M m 22 gm/cc scEfWINVENTORS SHiE/OAA/ R dew/(s VV/A z /AM A R550 Mir/f W United StatesPatent INCREASING THE DENSITY OF IRON POWDER BY ALTERNATE ROLLING ANDGRINDING Sheridan R. Crooks, Maple Heights, and William A. Reed, WestRichfield, Ohio, assignors to Republic Steel Corporation, Cleveland,Ohio, a corporation of New ersey Application June 11, 1954, Serial No.435,978

3 Claims. (Cl. 75-5) The present invention relates to increasing thedensity of iron powder by alternaterolling and grinding, whilecontrolling the flow characteristics thereof and while minimizing thework-hardening of the powder incident to the treatment thereof so thatthe powder as finally produced will be useful in powder metallurgy.

In the production of iron powder for the purpose of powder metallurgy,it is conventional to reduce some one or more of the compounds of iron,using some available reducing agent or agents, so as to leave the ironsubstantially all in metallic form, but usually in a physical formwherein a part at least thereof is sponge iron or analogous thereto inits physical characteristics. This is true, for example, whether theiron be reduced from an iron halide, such as ferrous chloride, by theuse of hydrogen or some similar reducing gas, or whether it be reducedfrom one or more of the oxides of iron by the use of some suitablereducing agent. Other compounds of iron, such as the formate, have alsobeen reduced by gaseous reducing agents.

In some instances, the temperatures used during the reduction aresufiiciently high, so that some or all of the iron so produced may be inthe form of sponge iron. Under other circumstances and particularly whenquite low temperatures are used, the iron is in the form of powder. Ineither case, i.e. Whether the iron be in the form of a sponge or in theform of powder, the bulk density thereof is substantially less than thatwhich is required commercially by the users of iron powder for powdermetallurgy. Thus, if the iron powder produced as aforesaid were useddirectly and Without any further treatment, molds in which the finalarticles are formed would have to have a very substantial overcapacityto accommodate the large volume of powder required in order that asulficient Weight thereof be provided for making a desired article.

A primary object of the present invention is to provide a method oftreating the powder and/or sponge iron produced as aforesaid inaccordance with the inventions of others and/or in accordance with theprior art, so as to increase the bulk density thereof to the valuedesired by fabricators of such iron powder, this desired bulk densityusually being, in practice, in the range of about 2.3 to about 2.5 gramsper cc. It is also required, in order to provide an iron powder whichwill be satisfactory to fabricators of powdered iron products, that theflow characteristics of the product shall be within certain desiredlimits.

Summarizing the present invention, there is hereby provided a method oftreating iron produced as aforesaid to provide an iron powder which willbe acceptable and useful in powder metallurgy. To do this, the originaliron material, which is usually at least in part in sponge form, iscomminuted by a first grinding step into the form of a powder having abulk density in the range of about 1.8 to about 2.2 grams per cc. Thisgrinding may be effected by any of the known types of grinding equipmentas more particularly hereinafter discussed, but is preferably accom-"ice plished by a disk-type attrition mill. The next step is to roll thepowder into the form of a frangible sheet by passing it between smoothsurfaced rolls, which are usually located with their axes in the samehorizontal plane. The frangible sheet thus produced has a bulk densityin the range of 3 to about 4 grams per cc. This frangible sheet is thenground in a second grinding step to produce iron powder approximatingthe final desired product. With the exception of a step which is often,but not necessarily, performed of screening out oversized particles tobring the powder within the particular requirements of individualmanufacturers, the product of the second grinding step aforesaid may beconsidered as the final product. In order to comply, however, with theparticular requirements of different fabricators of the powder, it isusually required that a screening or a separation step be effected withthe oversized particles to be returned to the process at a stage atleast as early as the second grinding step. It may, of course, bereturned to the process either to the first grinding step or to therolling step aforesaid.

The process above generally described is illustrated in the accompanyingdrawing, which comprises a flow sheet of the process.

The reduced iron supplied to the process is not suitable as iron powderfor use in fabrication of articles by conventional powder metallurgyprocesses without some further processing. The reason for this is thatin some instances at least, and depending upon the process by which theiron was reduced, it may be partially or wholly in the form of small orlarge chunks or porous sponge-like masses. In other instances, andparticularly where quite low temperatures are used in effecting thereduction, the iron may be in powder form. In either case, the bulkdensity of the iron is usually substantially less than that desired.Furthermore, when the material supplied to the present process is in theform of chunks or various different size masses of sponge-like iron,this material does not have the desired flow characteristics of ironpowder, which is presently required by commercial fabricators ofpowdered iron articles.

The first process step, therefore, is required to bring all the iron,irrespective of the shape or form in which it is received, to the formof a powder. This step is accomplished by the use of grinding equipment,which is per se known in the art. The preferred type of grinding is onewhere the comminution is effected by a shearing action rather than by acrushing action. Thus, the preferred type of equipment for carrying outthis step is a disk-type attrition mill, or, alternatively, an airattrition mill.

Experimental work on this step of the process has shown that it isfeasible to use a jaw crusher to effect an initial comminution of thelarger size lumps or chunks of sponge iron and then to use a ball millfor the final comminution operation. This manner of operation is,however, quite slow; and, furthermore, unless the size of the balls isvery carefully selected in respect to the hardness of the iron beinghandled, there results what is termed work-hardening of the product,which renders this product relatively less desirable from the point ofview of the fabricator of the powdered metal parts to whom the productis supplied. In general, it is desired to minimize work-hardening.

Another type of comminution apparatus which has been tried and which isoperative, but is relatively less desirable than the disk-type attritionmill or air attrition mill, is a hammer mill. Here again, the use of ahammer mill tends to work harden the iron powder.

The product of the first grinding operation aforesaid is an iron powderhaving a bulk density m the range of about 1.8 to about 2.2 grams percc.

The next step in the operation in accordance with the present inventionis to roll the powder produced by the first grinding'step as aforesaidinto the form of a'frangible sheet. To do this, it is usual to pass thepowder between smooth surfaced rollers, located with their axes both in'the same horizontal plane, and, of course, substantially parallel. Inpracticing this step, various size rollers have been used; for examplewhen the step was practiced on alaboratory scale, a pair of rolls about12 inches in diameter and with an axial length or face width of 2 incheswas used to produce a sheetwhich was about .08 inch to about .09 inch inthickness. There has also been used a pair of 12 inch diameter rollershaving an 8 inch face Width, to give a sheet about 0.15 inch to about0.25' inch in' thickness. Other sizes and dimensions of rolls could 'beused if desired. The critical limitations on this step do' not pertainto the diameter of the rollers, their face widthor the spacing betweenthe rollers at their closest approach to each other, but rather relateto the density of the rolled sheet. It has been found that the actualsheet thickness is not particularly important or critical, but is to agreat extent a function of the dimensions of the rolls and the spacingtherebetween. The density of the sheet, however, is quite important fromthe point of view of the present invention. It has been found thatirrespective of its thickness, the rolled sheet should have a density inthe range of about 3 to about 4 grams per cc. When such a sheet isproduced, irrespective of the thickness and/ or Variations in thicknessthereof due to possible springing of the rolls, the desired results inaccordance with the present invention are attained.

The next step in the process is to grind the frangible sheet, producedby the rolling step aforesaid, to powder form. The requirements as tothe type of grinding for this second grinding operation aresubstantially the same as those given above in respect to the firstgrinding step. It has been found that either or both the grinding stepsmay be performed using the preferred type of grinding equipment ormethod, i.e. attrition as distinguished from crushing; and preferablythat both steps should be effected using the preferred type of grinding.

The material resulting from this second grinding step will usually havethe desired bulk density, i.e. from about 2.3 to about 2.5 grams per cc.This material is usable without further treatment in the making ofarticles by powder metallurgy practices.

However, due to the practical requirement of measuring up to theparticular specifications of individual manufacturers of powdered metalparts, there is usually employed an additional step involving screeningout powder particles of greater than some particular screen size. Forexample, one manufacturer may require powder having a particle size ofIOU-mesh and finer and place a limitation on acceptable powder that itshall not contain more than one percent of material having a particlesize greater than 100-mesh (standard Tyler screen). Another manufacturermay place similar limitations, but with the limit at 65-mesh and arequirement that not more than one percent of the powder particles shallexceed this screen size. It has been found that if a 90-mesh screen isused and the oversize discarded or reprocessed, the product, i.e. thematerial passing through a 90-mesh screen will be satisfactory to many,if not all, of the fabricators of powdered metal parts.

Whatever size limits are used as aforesaid, it is contemplated inaccordance with the present invention that the over-size particlesseparated out by the screening step will be returned to the process atleast as early as the second grinding step and possibly tothe-rollingstep aforesaid. The accompanying flow sheet illustrates thisoversize material being returned'to the second grinding step, which is apreferred manner of practicing the method of the present invention.

As a further alternative embodiment of the process of the presentinvention, the product of the second grinding step may be furtherdensified, i.e. its bulkdensity may be further increased by additionalpairs of alternate rolling and grinding steps, each pair of which isrespectively similar to the rolling step aforesaid and'the secondgrinding step, it being understood, of course, that the final step ofthe process or the final step prior to the screening step above referredto will be a grinding step and not a rolling step.

There is hereinabove described and there is illustrated on a flow sheetbasis a preferred embodiment of the present process. There has also beenset out as the description proceeded certain alternatives which areusable in respect to certain of the preferred steps or certainalternative ways of performing these steps, in addition to the preferredmanner in which these steps are respectively performed. Otherequivalents will occur to those skilled in the art from the foregoingdescription. We do not wish to be limited, therefore, except by thescope of the appended claims, which are to be construed validly asbroadly as the state of the art permits.

We claim:

1. The method of treating iron, which was reduced to the metallic statefrom an iron compound under conditions such that the metallic ironproduced was not'melted or firmly sintered together, while preventingworkhardening and increasing the bulk density thereof, so as to providean iron powder having a bulk density and fio'w characteristics such thatit is useful in powder metallurgy, said method comprising the steps ofcomminuting the reduced iron solely by attrition to produce an ironpowder having a bulk density in the range of about 1.8 to about 2.2grams per cc., compacting the powder thus produced solely by rolling itbetween smooth surface rolls and without positively heating it toproduce a frangible sheet having a bulk density of about 3 to about 4grams per cc., and thereafter comminuting said frangible sheet solely byattrition to provide an iron powder haw ing a bulk density in the rangeof about 2.3 to about 2.5 grams per cc. and which is not substantiallyworkhardened as compared with the iron supplied to or that at anyearlier stage of the method aforesaid.

2. A method of increasing the volume Weight of metal powder withoutsubstantially increasing its grain size which comprises cold rolling thepowder in a layer ofsufficient thickness and under suflicient pressureto form a continuous layer and comminuting said layer to a powder.

3. A method of increasing the volume Weight of metal powder whichcomprises cold rolling the powder in a layer of sufiicient thickness andunder suflicient pressure to form a continuous layer and comminutingsaid layer to a powder.

References Cited in the file of this patent UNITED STATES PATENTS

1. THE METHOD OF TREATING IRON, WHICH WAS REDUCED TO THE METALLIC STATEFROM AN IRON COMPOUND UNDER CONDITIONS SUCH THAT THE METALLIC IRONPRODUCED WAS NOT MELTED OR FIRMLY SINTERED TOGETHER, WHILE PREVENTINGWORKHARDENING AND INCREASING THE BULK DENSITY THEREOF, SO AS TO PROVIDEAND IRON POWDER HAVING A BULK DENSITY AND FLOW CHARACTERISTICS SUCH THATIT IS USEFUL IN POWDER METALLURGY, SAID METHOD COMPRISING THE STEPS OFCOMMINUTING THE REDUCED IRON SOLELY BY ATTRITION TO PRODUCE AN IRONPOWDER HAVING A BULK DENSITY IN THE RANGE OF ABOUT 1.8 TO ABOUT 2.2GRAMS PER CC., COMPACTING THE POWDER THUS PRODUCED SOLELY BY ROLLING ITBETWEEN SAMOOTH SURFACE ROLLS AND WITHOUT POSITIVELY HEATING IT TOPRODUCE A FRANGIBLE SHEET HAVING A BULK DENSITY OF ABOUT 3 TO ABOUT 4GRAMS PER CC., AND THEREAFTER COMMINUTING SAID FRAGIBLE SHEET SOLEY BYATTRITION TO PROVIDE AND IRON POWDER HAVING A BULK DENSITY IN THE RANGEOF ABOUT 2.3 TO ABOUT 2.5 GRAMS PER CC., AND WHICH IS NOT SUBSTANTIALLYWORKHARDENED AS COMPARED WITH THE IRON SUPPLIED TO OR THAT AT ANYEARLIER STAGE OF THE METHOD AFORESAID.